1. Field of the Invention
The present invention relates to a face region estimating device, a face region estimating method, and a face region estimating program, for estimating a region of a face of a person existing on an image taken by a camera.
2. Description of the Related Art
Conventionally, a typical technique for estimating a face region (skin color region) of a person existing on an image taken by a camera such as a CCD camera was to extract from the image a region contained in a fixed range of values representing the skin color in hue, saturation and lightness (luminance) in color space to estimate the face region (skin color region).
As another technique, Japanese Published Unexamined Patent Application No. H09-44670 (Paragraphs 0011 to 0015, FIG. 1 to FIG. 3) discloses a technique for extracting a face region suited for a person's skin color as a candidate for extraction by generating the histogram of the hue value in the image and defining the hue value range of the skin color from the shape of the histogram.
As yet another technique, Japanese Published Unexamined Patent Application No. 2004-13768 (paragraphs 0020 to 0031, FIG. 1 to FIG. 8) discloses a technique for estimating that the skin color region is a person's face region by defining a skin color in a Munsell color system, extracting a skin color region from the image, comparing the skin color region with the feature amount of a preregistered individual's face to detect (identify) the face.
However, in the typical techniques for extracting from the image the region contained in the value range of hue, saturation and lightness showing a fixed skin color, it was necessary to adjust the white balance of the camera and to change the threshold of the color space (hue, saturation and lightness) whenever the illumination environment, for example, changes. That is, the technique for setting the threshold of the fixed color space had a problem of not being capable of extracting the person's face region (skin color region) because of the change in the illumination environment, for example. The technique had another problem in that it cannot simultaneously extract each person's face region when the image contained another person with a different skin color.
According to the technique disclosed in the Japanese Published Unexamined Patent Application No. H09-44670, each person's face region can be extracted by the histogram of the hue value even when the illumination environment, for example, changes and the image contains a person with a different skin color. However, because this technique detects the region having the same color distribution as the skin color, there is a problem in that the face region is erroneously detected.
The technique disclosed in the Japanese Published Unexamined Patent Application No. 2004-13768 has problems in that the image must have a high resolution to the extent the feature amount of the face can be extracted, and therefore the unit becomes expensive because of a camera having a high resolution and a high-capacity memory for memorizing the high resolution images, for example.
In view of the above-mentioned problems, it is an object of the present invention to provide a face region estimating device, a face region estimating method, and a face region estimating program capable of extracting each person's face region from an image taken by a camera without using a high resolution camera, even when the illumination environment, for example, is changed and persons of different skin colors are contained in the image.